This invention is concerned with r.f. sputtering of compounds in sputtering chambers.
For background of the field of the invention, reference may be had to U.S. Pat. No. 3,829,373 and to the disclosure of application Ser. No. 323,133 filed Jan. 12, 1973, now U.S. Pat. No. 3,884,787, whose disclosures are incorporated herein by reference. As a consequence, the details of the sputtering apparatus which are not directly concerned with this invention will not be included herein.
Briefly, the type of sputtering involved is a technique where the target is a member formed of the material which is to be sputtered onto a substrate, but is composed of more than a single element. In the practical example of this invention, it was desired to sputter cadmium sulfide onto a substrate of flexible, thin polyester resin material. As explained in the above-mentioned patent and application, the atmosphere in the sputtering chamber is argon, which provides the gas to be ionized and provides the plasma that furnishes the atoms that knock the molecules of cadmium sulfide out of the target and drive them to the substrate. Theoretically, if the stoichiometry of the target is perfect, then there is no need for any additional materials to be added to the atmosphere in the chamber. Actually, the vapor pressure of sulfur is less than that of cadmium, hence there is a tendency for the material of the target to be dissociated under bombardment and the sulfur to be driven off first. This causes sulfur to escape and be drawn off by the vacuum pump that is keeping the pressure in the chamber down.
Accordingly, it is known to provide a background gas to prevent the decomposition of the target material. This will be a gas that has a component which is the same as the most volatile of the elements making up the target. In the case of cadmium sulfide, this element is sulfur and the gas which is best used is hydrogen sulfide.
Very little work has been done in the sputtering field with targets and substrates that are larger than about six inches in diameter. As a matter of fact so far as known there has been nothing reported in the literature on the r.f. sputtering of multiple element compounds onto substrates which have the size which the applicant's substrate has. The applicant's substrate is a strip of polyester material, such as "Mylar" (Dupont trademark) which has a width of about 500 mm. and a length which is limited only by the size of roll that can be accommodated without interference inside of the chamber. Applicant uses a pair of targets which are each 240 mm .times. 560 mm in exposed surface. This should be contrasted with the usual target of about 30 square inches. Thus, the target of the applicant is of the order of 10 times larger than commonly used targets.
The conditions which exist in a sputtering chamber are not duplicated when the target and sputtered area are substantially increased in size. Thus, one cannot merely make everything bigger and expect to get a larger yield. Problems arise because of the large areas involved. The most important of these have to do with uniformity of deposit and stoichiometry, although many additional problems require solution of a nature not within the purview of this invention.
The need for background gas is common to sputtering machines with small targets as well as large targets. In the case of small sputtering apparatus, one merely admits the gas into the chamber and adjusts the pressure for best results. In the case of the larger apparatus, it was found that merely admitting the gas and attempting to adjust the pressure was not enough. Targets were pitting and cracking, deposits were not uniform, stoichiometry was not maintained. For example, if stoichiometry is perfect, the properties of the coating will not improve substantially with heat treatment.
The invention achieved excellent results by a structural change which enables the background gas to be admitted in a most unusual place. The gas is applied with the plasma-forming gas to the target directly over its surfaces as it is sputtering and only thereafter flows into the chamber. In this manner the disadvantages mentioned above have been eliminated.